Zoisite and clinozoisite are minerals that form during the regional metamorphism and hydrothermal alteration of igneous, metamorphic, and sedimentary rocks. In those environments they are found in massive form and as prismatic crystals in veins that cut schists and marbles. They are also found as crystals in pegmatites that form on the margins of igneous bodies. The two minerals are dimorphs - they share the same chemical composition but have a different crystal structure. Zoisite is the orthorhombic form of Ca2Al3(SiO4)(Si2O7)O(OH) and clinozoisite is the monoclinic form. The minerals have extremely similar physical properties and can be very difficult to tell apart in hand specimens unless the specimens are well-formed crystals. Clinozoisite forms a solid solution series with the mineral epidote in which iron can substitute for aluminum - "".